Heat pump HVAC units have been used for some time to heat and cool spaces that people occupy such as the interior of buildings. Heat pumps have also been used for other purposes such as heating water and providing heat for industrial processes. Heat pumps are typically more efficient than alternative heat sources, such as electrical resistance heating, because heat pumps extract heat from another source, such as the environment, in addition to providing heat produced from the consumption of electrical power. As a result, heat pumps often reduce energy consumption in comparison with alternatives.
More broadly speaking, a heat pump is a machine or device that transfers thermal energy from one location, at a lower temperature, to another location, which is at a higher temperature. Accordingly, heat pumps move thermal energy in a direction opposite to the direction that it normally flows. Thus, air conditioners and freezers are also types of heat pumps, as used herein. Some types of heat pumps are dedicated to refrigeration only, some types are dedicated to heat only, and some types perform both functions, for instance, depending on whether heating or cooling is needed at the time. Further, in some applications, the heating and the cooling are both put to beneficial use at the same time.
In many applications, heat pumps extract heat from air, such as outdoor air, when a heat pump is being used to provide heat. In other examples, heat pumps extract heat from air that is being cooled such as air in a freezer when the heat pump is being used to cool the freezer. When a heat pump is used to extract heat from outdoor air, if the outdoor air temperature is close to or below freezing, moisture in the air can be deposited onto the outdoor air heat exchanger of the heat pump forming frost on the heat exchanger. The same may occur on a heat exchanger used to cool a freezer or refrigerator, as other examples. Build up of frost on the heat exchanger can interfere with heat transfer from the air to the refrigerant in the heat pump. Specifically, frost can insulate the heat exchanger, or can even block air flow through the heat exchanger. To address this problem, heat pumps have been operated in a defrost mode during a brief defrost cycle, during which the heat exchanger is warmed to melt the frost.
For example, heat pumps that are used in an HVAC application to heat and cool a building, when being used in a heating mode, may interrupt the heating mode periodically to run a defrost cycle. In the defrost cycle, the heat pump may be operated in the cooling mode, except without the outdoor air fan running. In this mode, hot refrigerant gas is delivered to the outdoor air heat exchanger heating the heat exchanger and melting frost that has accumulated on the heat exchanger. After the defrost cycle has been completed, the heat pump returns to the heating mode until another defrost cycle is initiated.
In recent years, microchannel heat exchangers have replaced other types of heat exchangers in various applications including automobile air conditioning. Microchannel heat exchangers typically have a first header, a second header, and multiple cross tubes extending from the first header to the second header. See U.S. patent application Ser. No. 12/561,178, Publication 2010/0071868, for example. Usually, each of the multiple cross tubes directly connects at one end to the first header and each of the multiple cross tubes directly connects at the other end to the second header. Moreover, in microchannel heat exchangers, the first header is often parallel to the second header, the multiple cross tubes are often parallel to each other, the headers are often perpendicular to the cross tubes, and the multiple cross tubes typically each include multiple contiguous parallel refrigerant passageways therethrough (e.g., extending from the first header to the second header). These refrigerant passageways are typically smaller than refrigerant passageways in prior heat exchanger designs (e.g., tube and fin heat exchangers), which is the origin of the name “microchannel”. Furthermore, most microchannel heat exchangers include multiple fins between the cross tubes, and the fins are typically bonded to the cross tubes. Microchannel heat exchangers generally offer a relatively high effectiveness relative to their cost and the restriction that they provide, in comparison with prior heat exchangers used for similar purposes. Microchannel heat exchangers generally also require less refrigerant, in comparison with prior heat exchangers used for similar purposes, and are also generally smaller and lighter in weight than alternative heat exchangers providing equivalent performance.
Microchannel heat exchangers have also been used in place of other types of heat exchangers in residential air conditioning units. In heat pump HVAC units, however, it has been found that microchannel heat exchangers do not defrost as well as certain prior heat exchangers. For example, if during a defrost cycle, hot refrigerant gas is introduced into the first header and travels though the cross tubes to the second header, the second header and the ends of the cross tubes that are connected to the second header often have not gotten warm enough to melt all of the frost there within a desired amount of time. As a result, frost or ice may remain on this portion of the heat exchanger after the defrost cycle is ended, or it may be necessary to extend the defrost cycle and remain in the defrost mode for a longer time.
Microchannel heat exchangers have been know for years to offer performance advantages, particularly relative to cost, size, weight, and the amount of refrigerant that is needed, in comparison with other types of heat exchangers. A long-felt need has existed to use microchannel heat exchangers in HVAC applications, but attempts to use microchannel heat exchangers for outdoor air heat exchangers in heat pumps have failed due to problems defrosting this type of heat exchangers. Others have taken many different approaches to resolving these problems, but none of their efforts have been successful and no heat pumps have been marketed that use a microchannel heat exchanger for the outdoor air heat exchanger.
U.S. Pat. No. 4,407,137 (Hayes) concerns a method and apparatus for defrosting a heat exchanger (50) having multiple rows (52 and 54) of cross tubes (Abstract, FIGS. 1 and 2, col. 3, lines 7-31). In Hayes, a solenoid valve (92) is opened during the defrost cycle to allow the hot refrigerant gas to bypass the second row (54) of the heat exchanger to the first row (52) of the heat exchanger to better defrost the first row where most of the frost typically accumulates in the tube and fin type of heat exchanger shown (col. 4, lines 45-52). Hayes uses three vertical headers on one side of the heat exchanger (FIG. 1 and col. 3, lines 25-26), which include an intermediate header (70) connected with feeder tubes (64 and 66) to the two rows (52 and 54) of cross tubes of the heat exchanger (50). The intermediate header is connected to each of the other headers (60 and 80) by horizontal cross tubes (rows 52 and 54) that pass through vertical fins (58, FIG. 1) and by feeder tubes (62, 64, 66, and 68). In Hayes, the refrigerant delivered to the second header through solenoid valve 92 and refrigerant conduit (hot gas bypass line) 90, passes through the cross tubes of coil row 52 before reaching header 60 (analogous to the second header of various embodiments herein). Hayes does not teach or suggest passing refrigerant through a header (e.g., 60, 70, or 80) of heat exchanger 50 without also passing that refrigerant through the cross tubes of coil row 52.
In various applications, in the defrost mode, as hot refrigerant gas is delivered to the heat exchanger, a portion of this heat will be transferred to the environment surrounding the heat exchanger. In particular, heat may be transferred via convection to air around the heat exchanger. Heat that is transferred to the air is not available or is less available to defrost the heat exchanger, especially for portions of the heat exchanger that are physically below the location where the heat is transferred to the air. As mentioned, in prior heat pumps, the outdoor air fan was typically turned off during the defrost cycle, which avoids heat loss to the surrounding air through forced convection. Natural convection still occurs, however, under such circumstances, carrying the hot air and heat upward where the heat is lost to the environment. For example, air heated by the heat exchanger can travel upward through the fan, pushed up by buoyancy forces from denser colder air, and colder air tends to flow through the heat exchanger to replace the warm air that has risen. This colder air flowing through the heat exchanger continues to cool the heat exchanger, cooling the refrigerant and taking heat away from the intended purpose of melting the frost. As a result, frost has remained on the heat exchanger, particularly on the lower portion of the heat exchanger, after a defrost cycle is completed, and it has been necessary to extend the defrost cycle and remain in the defrost mode for a longer time in order to defrost a heat exchanger completely or adequately.
Extending the defrost cycle in HVAC applications, for example, is undesirable because the heat pump delivers cold air to the space during the defrost cycle, which may lower the temperature in the space significantly below the thermostat set point temperature, may cause a cold draft and discomfort to the occupants of the space during the defrost mode, may cause the occupants of the space to believe that the heat pump is not working properly, or a combination thereof, for instance. Extension of defrost cycles and less effective defrost cycles may be undesirable in other applications (besides HVAC) as well, among other things, because heating or cooling is unavailable during the defrost cycle and because energy used during the defrost cycle does not contribute to the heating or cooling that is intended to be produced by the heat pump.
As a result, needs or potential for benefit or improvement exist for defrost cycles for heat pumps and methods of defrosting heat exchangers of heat pumps that are more effective, that direct hot refrigerant gas to areas of the heat exchanger that otherwise would not get warm enough, that take less time to complete, that work effectively with microchannel heat exchangers, or a combination thereof, as examples. In addition, needs or potential for benefit or improvement exist for defrost cycles for heat pumps, and methods of defrosting heat exchangers, that reduce the amount of heat loss to the air from the heat exchanger during the defrost cycle, that reduce natural convection during the defrost cycle, or a combination thereof, as examples. Needs and potential for benefit or improvement also exist for heat pumps and methods of defrosting heat exchangers that that are inexpensive, that can be readily manufactured, that are easy to install, that are reliable, that have a long life, that are compact, that are efficient, that can withstand extreme environmental conditions, or a combination thereof, as examples.
Further, needs or potential for benefit or improvement exist for methods of controlling, manufacturing, and distributing such heat pumps, HVAC units, buildings, systems, devices, and apparatuses. Other needs or potential for benefit or improvement may also be described herein or known in the HVAC or heat pump industries. Room for improvement exists over the prior art in these and other areas that may be apparent to a person of ordinary skill in the art having studied this document.